This invention is concerned with altering signals, such as an optical carrier, according to various other signals and measurands of interest so that the signal or measurand of interest can be transmitted or measured.
In many sensing systems, most notably fiber optic ones, a sensing element modifies some characteristic of input optical signal by changing some parameter of the optical system. Sensing elements can either be constructed of fiber (intrinsic), or can be some external fiber-coupled transduction element (extrinsic). The performance of most sensing elements (e.g. transducers, modulators, etc.) depends on an appropriate state of polarization of the input light. The signal carried by the light from the sensor can be degraded or completely eliminated by an inappropriate input state of polarization, this fading being commonly known as polarization induced signal fading. Signal fading is encountered when a conventional low-birefringence single mode optical fiber is used to deliver the optical input from a source to a sensor. The state of polarization at the input to the sensor is unpredictably varying due to environmental perturbations, and because of randomly distributed birefringence along the fiber.
Several schemes have been devised to overcome the effects of polarization fading, including automatic polarization tracking, polarization diversity detection schemes based on output state of polarization selection, and use of polarization maintaining fiber. However, except for use of expensive polarization maintaining fiber, these techniques all rely on active control.
Recently, a new scheme allows passive, polarization-independent operation of a fiber interferometer. However, the new scheme can only be implemented with unpolarized sensing elements and only in a reflective configuration. This scheme cannot be used with polarized sensing elements. Also, this scheme requires that the signal from the sensing element return on the same fiber as the light from the source--an undesirable because of source light backscatter (noise) co-propagating with signal light.
As stated above, many fiber-optic and other transducers are polarization sensitive. If the incoming light does not reach the transducer in the correct polarization, then the light may be strongly attenuated after passing through the transducer (in the case of a polarizer), or the transducer may not have any useful effect on the light passing through, or some combination of both. Examples of polarization-sensitive transducer configurations include interferometric ones (Mach-Zender, Michelson, Fiber Fabry-Perot), polarimetric, waveguides (e.g. lithium niobate phase modulators), and Faraday rotator based sensors. All of these sensor configuration are sensitive to the input state of polarization and can be constructed to maintain a preferential input state of polarization.